1. Technical Field
This invention relates to a composition for forming a porous film having improved dielectric properties, adhesion, coating uniformity, mechanical strength, and minimized moisture absorption, a method of manufacturing a porous film, a porous film, an interlayer dielectric film, and a semiconductor device having a porous film built therein.
2. Background Art
In the progress of semiconductor integrated circuits toward higher integration, an increase of interconnect delay time caused by an increase of wiring capacitance which is a parasitic capacitance between metal wires becomes a barrier in enhancing the performance of semiconductor integrated circuits. The interconnect delay time, also known as RC delay, is in proportion to the resistance of metal wiring multiplied by the wiring capacitance.
Then, in order to shorten the interconnect delay time, the resistance of metal wiring or the wiring capacitance must be reduced.
A reduction of wiring capacitance is effective for preventing higher integration semiconductor devices from incurring an interconnect delay, enabling higher speed operation and suppressing power consumption.
As the means of reducing the wiring capacitance, it is believed effective to reduce the relative dielectric constant of an interlayer dielectric film formed between metal wirings. For the dielectric film having a low relative dielectric constant, porous films have been investigated as a substitute for prior art silicon oxide films. Among those films which can have a relative dielectric constant of 2.0 or less, porous films are almost only one class of practically applicable films. A number of methods for forming porous films have been proposed.
A first method of forming porous film involves synthesizing a siloxane polymer precursor containing a thermally unstable organic component and applying a solution of the precursor onto a substrate to form a coating. The coating is then heat treated for decomposing and volatilizing off the organic component, with a multiplicity of pores being left behind the volatilized component.
In a second method of forming porous film, a wet gel is formed by applying a silica sol solution onto a substrate or by CVD process. The silica sol is subjected to condensation reaction to form a porous film while volume shrinkage is restrained by precisely controlling the rate of evaporation of the solvent from the wet gel.
A third method of forming porous film involves applying a solution of silica fines onto a substrate to form a coating and firing the coating. In the consolidated coating, a multiplicity of pores are formed between silica fines.
As a fourth method of forming porous film, JP-A 2000-44875 proposes a porous film-forming composition comprising (A) a component of the formula: R′nSiO(OR″)4-n wherein R′ is a monovalent organic group, R″ is a monovalent hydrocarbon group and n is an integer of 0 to 2, (B) a metal chelate compound, and (C) a compound having a polyalkylene oxide structure.
These methods, however, have notable drawbacks.
The first porous film forming method suffers from an increased cost because a siloxane polymer precursor solution must be synthesized. Additionally, since the precursor solution is applied to form a coating, an amount of silanol groups remain in the coating, which gives rise to a degassing phenomenon that moisture or the like evaporates in the subsequent heat treatment step. The porous film can be deteriorated in quality by moisture absorption.
The second porous film forming method needs a special applicator apparatus capable of controlling the rate of evaporation of a solvent from a wet gel, adding to the cost. Since more silanol is left on surfaces of pores, the film as such is hygroscopic and susceptible to substantial quality deterioration. Thus surface silanol must be silylated, which complicates the process. In case a wet gel is formed by the CVD process, a special CVD system which is different from plasma CVD systems customarily used in the semiconductor process is needed, adding to the cost as well.
In the third porous film forming method, pores of a very large size are formed because the pore size is dictated by the deposition structure of silica fines. It is then difficult for the porous film to have a relative dielectric constant of 2.0 or less.
In the fourth porous film forming method, the metal chelate compound (B) is essential for improving the compatibility between components (A) and (C) and making uniform the thickness of a coating after curing. Undesirably, component (B) complicates not only the composition, but also the manufacturing process, and increases the cost. There is a need to have a composition which can form a uniform solution absent a chelate component and whose coating is flat even after curing.
For all these methods, a lowering of mechanical strength of porous films themselves is a problem. To maintain a practically acceptable level of strength, it would be effective that fine pores having a narrow size distribution be dispersed uniformly in the film.
Now attention is drawn to a porous silica material prepared by hydrolytic polycondensation of a silica precursor in the presence of micelle since it is characterized by a nanometer pore size, a narrow pore size distribution, a high regularity structure, and a very high mechanical strength for porous material. It is expected to find application as a carrier for various catalysts and an insulating material having a low dielectric constant. The porous silica material is generally prepared by forming a W/O type micelle in an aqueous solvent, causing a hydrolyzate of alkoxysilane dissolved in the aqueous layer to condense and crosslink to form a silica skeleton, and thereafter removing the surfactant that has formed the micelle, thereby introducing pores in the silica skeleton. If the silica skeleton has not been densified at this point, the film can be collapsed during the surfactant removal step, failing to effectively introduce pores. Then, the silica skeleton is generally formed by prolonging high-temperature heating under such conditions as to prevent evaporation of water or by performing long-term ripening under highly alkaline conditions.
For the application of porous silica material to low dielectric constant material for electronic use, attention is paid to the low dielectric constant expected from a high film strength and high porosity. It is urgently required to solve outstanding problems including the shelf stability of a film-forming composition, a process time, and acid and alkali leftovers.
As mentioned above, prior art materials suffer from problems such as the deterioration of film quality by heat treatment and increased costs. Another problem is the difficulty to reduce the dielectric constant of a porous film because the pore size is increased during film formation. A further problem is that when a prior art porous film is incorporated as a dielectric film for multilayer interconnections in semiconductor devices, the film fails to provide a mechanical strength necessary for the manufacture of semiconductor devices.
If the porous film used as a dielectric film for multilayer interconnections in semiconductor devices has a high relative dielectric constant, there arises a fatal problem that the RC delay in the multilayer interconnections of semiconductor devices is increased, so that no improvements in the performance of semiconductor devices (toward higher speed and lower power consumption) are achievable. The low mechanical strength of the porous film has a negative impact on the reliability of semiconductor devices.